In general, performance requirements of radio frequency (RF) transmitter circuits may be high and may concern e.g. noise generation, susceptibility to outside interference, linearity in the operational range, etc. The radio frequency performance requirements of a multi-carrier transmitter are generally extremely high. To meet such requirements various solutions may be utilized (alone or in combination).
For example, to have a possibility to meet requirements on linearity, the output power amplifier (PA) may need to be linearized (e.g. by means of a linearization loop).
Furthermore, to have a possibility to meet requirements on susceptibility to interference, the PA and the linearization loop may need to be shielded/isolated from interfering signals of the antenna port (e.g. signals picked up by the antenna and/or internal signals reflected at the antenna port).
One possible solution to the problem of isolating the PA is to place a (single or double) circulator between the PA output and the antenna port as exemplified in EP 1432132 A1 and in U.S. Pat. No. 7,092,684 B2.
One problem with such solutions is that inter-modulation (IM) distortion generated by or associated with the circulator(s) cannot be handled by the linearization loop.
Nevertheless, such solutions fill their purpose well enough for single carrier transmitters. Single carrier transmitters typically have high PA-efficiency and any inter-modulation (IM) distortion in such transmitters is typically associated with the circulator(s). However, the inter-modulation distortion is not a serious issue for such transmitters since it worsens only the spectrum mask, the requirements of which are typically not difficult to fulfill.
In multi-carrier (MC) transmitters, however, peaks of the inter-modulation distortion will not only fold close to the respective carriers but also to frequencies that equal any multiple of the carrier spacing. Thus, the inter-modulation distortion is a serious issue for such transmitters, and need to be dealt with if it should be possible to fulfill the performance requirements.
Therefore, there is a need for improved circuits and methods that reduce interference and/or improve linearity in radio frequency transmitter circuits.